When you turn on your main light, you raise the quantity of light on the subject. The area behind the subject, however, will then appear darker in comparison. To attain a balanced exposure, this means you will need to use a background light to increase the light levels in the background. Using a background light may seem pretty simple. However, using it correctly can be a challenge for new photographers.
If you want the background to appear as it does normally to your eyes, you will need to add the same amount of light that you used on the subject’s face (if your main light reads f/8, the light on the background should read f/8). Of course, there’s no reason the background has to match “reality.” You can easily brighten or darken the background’s appearance by increasing or decreasing the amount of light on it relative to the main light source. (Note: If you paint your background a medium tone, you can darken it to near black or brighten it to near white by changing the amount of light you add.)
Backgrounds with strong lines look best when illuminated with a soft, even light. This is achieved by using a bounce light attachment to soften the light or using your flash without the parabolic reflector. If you are working with a background without distinct lines, you might want to create a hot spot behind the subject. This is done with a parabolic reflector, which focuses the light onto a spot behind where the subject will be positioned. The reflector allows the light to spread softly, becoming darker as it moves outward from the center. I prefer this style of background light because it helps to separate the subject from the background. Also, it can help you to direct the viewer’s eye right where you want it.
The height and angle of the background light can also change the characteristics of the light. When the light is directly behind the subject and aimed at the background, the light beam will create a circular pattern of light. This will ring the subject and fall off as it moves outward from the center. You can also lower the light and angle it upward to create a fan-shaped beam of light. This allows for better separation than placing the light right behind the subject. You can also put the backgorund light on the side or top of the background and have the beam of light come in from a different angle to achieve different looks with the same background.
Before setting up your lights, consider the tone of your backdrop. Medium to dark-toned backgrounds can be lit similarly. White backgrounds need to be approached differently; if you are not careful, a white wall behind a subject can reflect light from all around the subject’s head back into the lens. If you’re shooting digitally, meter the white area and adjust your lighting so that it reads ½ stop less than the main light to ensure it is not reflecting into your lens and compromising your images. You can create a simple action to brighten and blur your white wall/background in postproduction.
We repaint the white floor in our studio every two business days. Because of the foot traffic, plus the Harley and the Viper (these are popular props for our senior sessions), the floor looks terrible most of the time. We simply run an action that brightens and blurs the floor to do away with this problem in the final portraits.